For measuring one or, more particularly, a plurality of target points, numerous geodesic measurement devices have been known since ancient times. In this case, distance and direction or angle from a measuring device to the target point to be measured are recorded and, in particular, the absolute position of the measuring device with reference points possibly present are detected as spatial standard data.
Generally known examples of such geodesic measurement devices include the theodolite, tachymeter and total station, which is also designated as electronic tachymeter or computer tachymeter. One geodesic measuring device from the prior art is described in the publication document EP 1 686 350, for example. Such devices have electrical-sensor-based angle and distance measuring functions that permit direction and distance to be determined with respect to a selected target. In this case, the angle and distance variables are determined in the internal reference system of the device and, if appropriate, also have to be linked with an external reference system for absolute position determination.
In many geodesic applications, points are measured by specially configured target objects being positioned there. The latter usually consist of a plumb staff with a targetable marking or a reflector for defining the measurement path or the measurement point. A central geodesic measurement device can therefore also measure a relatively large number of target objects, although this necessitates the identification thereof. In the case of such measurement tasks, for controlling the measurement process and for defining or registering measurement parameters, a number of data, instructions, speech and further information are transmitted between target object—more particularly a handheld data acquisition device at the target object—and central measuring device. Examples of such data include the identification of the target object, inclination of the plumb staff, height of the reflector above ground, reflector constants or measurement values such as temperature or air pressure.
Modern total stations as geodesic measurement devices additionally have microprocessors for digital further processing and storage of detected measurement data. The devices generally have a compact and integrated design, wherein coaxial distance measuring elements and also computing, control and storage units are usually present in a device. Depending on the expansion stage of the total station, motorization of the targeting or sighting device and means for automatic target seeking and tracking can additionally be integrated.
Examples of tracking devices for geodesic devices for tracking a reflector used as a target are described in EP 0 465 584, JP 05322569, JP 07103761 and DE 195 28 465.
Devices for automatic—more particularly coarse—target seeking use, for example, a vertically and/or horizontally fanned-out laser beam emitted by the geodesic device, e.g. theodolite. If, during the movement of the theodolite about the vertical or tilting axis, the reflector arranged at the target station is impinged on and, consequently, part of the laser beam is retroreflected, a signal is generated by a receiving photodiode arranged in the telescope of the theodolite, with the aid of which signal the drive of the telescope is then stopped. What is advantageous in this case is that the system possibly reacts to all reflectors situated in the field of view, thus e.g. also to further target units that are present, but not of interest, or disturbing reflectors such as reflective films, reflectors of vehicles, window panes or similar articles.
The patent specification CH 676 042 discloses a device having a fan-like transmitter and receiver, which device is accommodated in a rotatable measuring head. The transmitting unit emits light pulses in a light fan, and the reflected pulses are correspondingly evaluated with regard to angle information.
A development of the above target seeking device for coarsely determining the target coordinates is described in CH 676 041. This involves a combination with an optoelectronic device for fine measurement. The actual target seeking device spans two mutually perpendicular fans used to coarsely measure the position of the target point two-dimensionally, and the subsequent fine measurement by means of the second device can then be carried out without the target seeking process.
In most cases, the seeking process is manually supported by means of radiotelephony or radio data transmission. In the case of the device described in the document DE 197 334 91, an additional optical receiving unit fitted to the target object is used to check whether the seeking beam of the tachymeter impinges on the target object. If the seeking fan is received, then the target object communicates a hit message to the tachymeter by means of radio data transmission, whereupon the seeking is stopped and fine targeting is effected. In particular, in this case, after receiving the first hit message, the tachymeter begins a rotation in the opposite direction. If the light beam from the transmitter unit impinges on the receiver for a second time, a feedback message is in turn reported to the tachymeter. On the basis of the time that has elapsed between the two hits in the electronic unit of the tachymeter, a position of the target object is determined.
A prism device having an additional receiving device for the optical transmission from a geodesic measuring device to the prism device is presented in U.S. Pat. No. 6,295,174. In this case, radiation is coupled out by means of an optical path from the region of the reflector and transmitted to a receiving area of the receiver, which is arranged axially parallel. The reception state is indicated by two LEDs that emit light in different colors.
EP 1 734 336 discloses a measurement system having a target unit, which has a reflector and also an optical receiver and transmitter. Said document proposes using the optical transmitter of the target unit, inter alia, for supporting the automatic target seeking process. Thus, after receiving the seeking or measurement radiation, the target object can communicate the dedicated identification, such as, for example, the reflector number or the reflector type, back to the measurement station with the aid of the transmitter of the target unit. The measurement station can thus identify the target object sought and configure itself optimally with regard to the target object.
EP 1 573 271 also discloses a target unit having an optical transmitter, wherein—after receiving measurement radiation from a measurement device—the target unit sends back an optical signal, on which the dedicated identity of the target unit is modulated.
One significant disadvantage of the devices known hitherto is an inadequate robustness against incorrect identification of the target unit in the case of a plurality of target units situated in the field of view. In some devices in the prior art there is likewise the risk of inadvertent interpretation of a reflective foreign object as the target object. Moreover, in some known devices in the prior art, the complex communication of the target unit identities from the respective target units to the measurement device proves to be disadvantageous.
If e.g. a plurality of target units are set up which—e.g. in accordance with the technical teaching from EP 1 573 271—each communicate their dedicated identity to the measurement device in reaction to detection of a seeking beam, then the measurement station receives a respective identity communicated from each of the targets. If, however, only one target unit is sought in this case, then a disadvantageously high data processing complexity is required on the part of the measurement device in order to evaluate the multiplicity of responses of the respective target units and to filter and identify the target unit actually sought from the responses of all the target units.